U.S. Pat. No. 3,257,641 issued to Patsy C. Campana and Thomas T. Chrysler on June 21, 1966 teaches the idea of equipping an emergency vehicle with a radio transmitter which, when actuated by the vehicle's occupants, causes the intersection's traffic control system to preempt the traffic signals at the intersection. The Campana patent proposes the use of a 255 megacycle, tone modulated transmitter in the emergency vehicle. When the vehicle's occupants press a pushbutton, this transmitter sends out a radio signal modulated with an audio tone. A receiver mounted on the traffic light, in response to receipt of the proper frequency and tone combination, causes a preemption controller unit to preempt the traffic signals at the intersection by forcing the intersection's traffic control system to present red signals in all directions.
The Campana patent proposes utilizing multiple tones in the transmitter to enable an emergency vehicle to control several functions, and U.S. Pat. No. 3,638,179 issued to Edward T. Coll et al. on Jan. 25, 1972 discloses such a system. The occupant of the vehicle sets a switch to indicate the direction of travel, and that switch selects a tone which corresponds to the direction selected. A radio signal modulated with the selected tone is then produced. A receiver mounted on the traffic light responds differently to different tones, causing the traffic control system at an intersection to present a green light in the direction of the approaching emergency vehicle and a red light in all other directions. U.S. Pat. No. 4,443,783 issued to Wilbur L. Mitchell on Apr. 17, 1984 discloses a transmitter that modulates with dual tones and thus provides improved noise and invalid (or counterfeit) signal immunity.
An improved preemption system is disclosed in U.S. Pat. No. 4,228,419 issued to George P. Anderson on Oct. 14, 1980. Here, a directional transmitter on the emergency vehicle actually sends a message byte (to number) to the receiver at each intersection. One particular message byte (or number) informs the receiver that the vehicle's siren and warning lights are in operation, and the receiver responds by preempting the intersection for a fixed, predetermined length of time. Another particular message byte (or number) informs the receiver that the vehicle's occupants have actuated a manual preemption switch, and the receiver responds by preempting the intersection for as long as the manual switch remains actuated. So two distinct preemption functions are possible. The patent teaches that other distinct functions may be selected by having the occupant of the vehicle depress one or more keys on a keyboard. This patent teaches that a unique code can be assigned to each vehicle and transmitted so that the receiver at each intersection can forward to police headquarters the identity of the signalling vehicle and its location. This patent teaches that the direction of emergency vehicle travel can be determined through the use of multiple directional receiving antennas, one for each possible emergency vehicle approach direction, with each directional receiving antenna having its own receiver, demodulator, and message byte decoder. At a four-way intersection, assumedly four receivers, demodulators, and decoders would be required.
Optical transmitters have been utilized in the design of preemption systems. U.S. Pat. No. 4,230,992 issued to John A. Munkberg on Oct. 28, 1980, for example, discloses a system utilizing separate north-south and east-west optical signal receivers designed to receive optical pulses whose energy content exceeds a predetermined threshold level, rejecting all pulses not generated at one of two predetermined, precise pulse repetition rates. This patent also teaches the use of first and second optical pulse repetition rates to signal respectively lower and higher priority vehicles, and it discloses a mechanism that preempts an intersection in favor of the higher-priority vehicle when two vehicles having different priorities approach the intersection simultaneously from different directions.
Optical systems such as that just described are highly directional--light cannot flow around trucks, trees, and other such obstacles. Weather conditions such as fog can interfere with the operation of an optical system.
All of the above systems initiate a preemption in response to the receipt of a standard signal selected from a small set of valid preemption signals which are the same for all the emergency vehicles within a given city. If a transmitter is stolen, there is no simple way to cause intersections to ignore the preemption signals from that one stolen transmitter without also causing them to ignore the preemption signals from all the other transmitters in the city.
Except through the use of multiple directional receivers, these systems are unable to process multiple signals receive simultaneously from multiple vehicles and to select intelligently which vehicle should gain preemption. These systems cannot utilize a single omni-directional receiving antenna without the possibility that one signal will drown out another or the two signals will interfere with each other and prevent either from being received.
These systems also determine the direction from which an emergency vehicle approaches an intersection only through the use of multiple directional receiving antennas or, if a single omni-directional antenna is used, through manual depression of a directional pushbutton by the vehicle's occupants. There is no way in which the direction of emergency vehicle travel can be determined fully automatically except through the use of multiple directional receiving antennas or actuation of manual switches by vehicle occupants at each intersection.